This is an improvement of the invention disclosed and claimed in U.S. patent application Ser. No. 331,930 entitled, "Center Compensating Master Cylinder", filed on Apr. 3, 1989, and assigned to the common assignee.
Typical master cylinders in use in automotive vehicles for many years are of the dual or tandem type; that is, they have two pressurizing chambers in a common bore having a closed end and an open end, with brake fluid being pressurized by a primary pressurizing piston and a secondary pressurizing piston arranged in tandem in the bore. One of the chambers is typically connected to one brake circuit, such as one containing the vehicle front brakes, and the other chamber is typically connected to another brake circuit containing the vehicle rear brakes. At times other circuit arrangements, and even additional pressurizing chambers, are used.
Such a master cylinder usually has a pair of holes in the main body adjacent each of the primary and secondary pressurizing piston seals The holes are connected with the master cylinder reservoir and extend to the bore. One of the holes is a compensation port located behind the seal of the associated piston. The other hole is a bypass hole positioned in front of the seal of the associated piston. When the master cylinder is actuated, the seals move across their respective bypass holes so that these holes are no longer fluidly connected with the pressurizing chambers, and the brake fluid in those chambers and the brake circuits connected with them can be pressurized to actuate the brakes. These seals are usually cup-like or V-block seals with lips which are subject to seal extrusion into the bypass holes under some conditions, resulting in damage to the seal and loss of full functioning pressure generation for the brake circuit associated with the damaged seal. In normal service braking operation, there is little pressure in the pressurizing chambers until the bypass holes are closed, so extrusion into those holes is minimized. However, with the advent of systems variously referred to as anti-lock, anti-skid, wheel lock control, or anti-block systems the modulation of brake pressures in the brake circuits when that type system is operating can cause rapid travel of the secondary pressurizing piston seal back and forth across the front bypass hole. That is the bypass hole associated with the forward pressurizing chamber in the master cylinder bore.
The structure disclosed in the above noted patent application and embodying the invention claimed therein eliminated the usual front bypass hole over which the secondary pressurizing piston pressure seal must pass, thereby obviating the possibility of damage to that seal because of extrusion into such a bypass hole. That invention provided an internal path substantially along the center of the secondary pressurizing piston for brake fluid compensation and bypass for the secondary pressurizing chamber and its brake circuit, this being referred to herein as center compensation. That structure included a valve located on the secondary pressurizing piston within the master cylinder bore. The valve was open while the master cylinder was at the rest or released position, and was closed by actuating movement of the secondary pressurizing piston. It reopened when the secondary pressurizing piston was being returned to its rest position. The primary pressurizing chamber was compensated in the same manner as before, with the piston cup seal passing over the bypass hole during each master cylinder actuation operation and each release operation. Each compensation path was connected to its own reservoir or reservoir compartment.